As a system for inspecting a circuit board, there have been known a pin-contact system and a non-contact system. As shown in FIG. 1, the pin-contact system is configured to inspect electrical continuity between both ends of a conductive pattern as an inspection object by bringing two pin probes directly into contact with the ends, respectively, and applying a current to one of the pin probes so as to determine a resistance value of the conductive pattern from a detected voltage at the other pin probe.
This pin-contact system has an advantage of a high signal-to-noise (SN) ratio because of the pin probes contacted directly with the conductive pattern.
On the other hand, in case of inspecting a fine-pitch board, it is fundamentally difficult to set up the pin probes only to a conductive pattern as an inspection object, and it is increasingly hard to secure an adequate positioning for bringing the pin probes into contact with the aimed pattern. Due to the necessity for keeping in the contact state, it is also difficult to maintain the initial accuracy of the pin probes themselves, resulting in undesirably increased running cost arising from replacements of the pin probes.
As shown in FIG. 2, the other non-contact/contact combined system is configured to apply an inspection signal including an alternating current (AC) component with making one pin probe contact directly with one end of a conductive pattern as an inspection object (or through a capacitive coupling in a non-contact manner) and detect the inspection signal through a capacitive coupling at the other end.
The non-contact/contact combined system allows at least one of the pin probes not to be contacted with a pattern wire or the conductive pattern. This provides a relatively wide acceptable range of positioning accuracy for the pin probe, which makes it possible to use the pin probe commonly for a plurality of pattern wires, and thereby to reduce the number of pin probes. Furthermore, since the pin prove is free from any wear, the combined system is effective for a board with a fine wiring pattern.
However, the non-contact/contact combined system has a small value of coupling capacitance and a high impedance (from several MΩ to several GΩ). Thus, this system cannot detect any defective portion having a resistance ranging from about 10Ω to about 100Ω.
As a result, due to the property including high impedance despite having many advantages, the non-contact/contact combined system has been actually implemented only for a board with an extremely narrow pitch not to allow pin probes to be adequately set up thereon. Thus, the required high accuracy in the pin probes and a jig thereof has been an obstacle in the effort to facilitate the cost reduction in the non-contact/contact combined system.
It is therefore an object of the present invention to provide a continuity inspection apparatus capable of inspecting any electrical conductivity not only under a high resistance but also under a low resistance by making a capacitance provided in the non-contact system generate a resonance in oscillations of a circuit formed on a board to reduce the impedance of the circuit.